Semiconductor device having multiple wells

ABSTRACT

A semiconductor device includes a substrate and a gate structure over the substrate. The semiconductor device includes a source in the substrate on a first side of the gate structure. The semiconductor device further includes a drain in the substrate on a second side of the gate structure. The semiconductor device further includes a first well having a first dopant type, wherein the first well contacts at least two surfaces of the source. The semiconductor device further includes a second well having the first dopant type, wherein the second well contacts at least two surfaces of the drain. The semiconductor device further includes a deep well below the first well and below the second well, wherein the second well extends between the first well and the deep well. In some embodiments, the deep well has a second dopant type, and the second dopant type is opposite the first dopant type.

PRIORITY CLAIM

The present application is a divisional of U.S. application Ser. No.17/203,323, filed Mar. 16, 2021, which is a divisional of U.S.application Ser. No. 15/918,747, filed Mar. 12, 2018, now U.S. Pat. No.10,957,772, issued Mar. 23, 2021, which is a continuation of U.S.application Ser. No. 14/317,185, filed Jun. 27, 2014, now U.S. Pat. No.9,917,168, issued Mar. 13, 2018, which is a continuation-in-part of U.S.application Ser. No. 13/928,971, filed Jun. 27, 2013, now U.S. Pat. No.9,583,618, issued Feb. 28, 2017, which are incorporated herein byreference in their entireties.

BACKGROUND

A metal oxide semiconductor field effect transistor (MOSFET) whichincludes lightly doped drain (LDD) regions includes a capacitance formedbetween a gate electrode and LDD regions on source and drain sides ofthe MOSFET. Spacer layers formed over sidewalls of the gate electrodeact as an insulating material between the conductive features of thegate electrode and the LDD regions. A first capacitor Cgs is formedbetween the gate electrode and the LDD region on the source side of theMOSFET, and a second capacitor Cgd is formed between the gate electrodeand the LDD region on the drain side of the MOSFET.

An amount of capacitance in the first capacitor Cgs and the secondcapacitor Cgd is determined by a dopant concentration in the LDD regionsand by an amount of overlap of the LDD regions with the gate electrode.As the dopant concentration of the LDD regions increases, a conductivityof the LDD regions increases resulting in an increase in capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments are illustrated by way of example, and not bylimitation, in the figures of the accompanying drawings, whereinelements having the same reference numeral designations represent likeelements throughout. It is emphasized that, in accordance with standardpractice in the industry various features may not be drawn to scale andare used for illustration purposes only. In fact, the dimensions of thevarious features in the drawings may be arbitrarily increased or reducedfor clarity of discussion.

FIG. 1A is a cross-sectional view of a MOSFET in accordance with one ormore embodiments;

FIG. 1B is a dopant concentration profile across the MOSFET of FIG. 1Ain accordance with one or more embodiments;

FIG. 2 is a schematic diagram of a converter including the MOSFET ofFIG. 1A in accordance with one or more embodiments;

FIGS. 3A-3E are cross-sectional views of n-type MOSFET (NMOS) devices inaccordance with one or more embodiments;

FIGS. 4A-4F are cross-sectional views of n-type MOSFET (NMOS) devices inaccordance with one or more embodiments;

FIGS. 5A-5D are cross-sectional views of p-type MOSFET (PMOS) devices inaccordance with one or more embodiments;

FIG. 6 is a flow chart of a method of making a MOSFET in accordance withone or more embodiments;

FIGS. 7A-7H are cross-sectional views of a MOSFET during various stagesof production in accordance with one or more embodiments; and

FIG. 8 is a flow chart of a method of forming a variable thickness gatedielectric layer of a MOSFET in accordance with one or more embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are examples and are not intended to belimiting.

FIG. 1A is a cross-sectional view of a metal oxide semiconductor fieldeffect transistor (MOSFET) 100 in accordance with one or moreembodiments. MOSFET 100 includes a substrate 102 a source region 104 inthe substrate and a drain region 106 in the substrate. MOSFET 100further includes a gate structure 110 over substrate 102 positionedbetween source region 104 and drain region 106. Gate structure 110includes a variable thickness gate dielectric 112 over substrate 102 anda gate electrode 114 over the gate dielectric. Gate structure 110 alsoincludes spacers 116 over substrate 102 covering sidewalls of variablethickness gate dielectric 112 and gate electrode 114. MOSFET 100 alsoincludes contacts 130 configured to provide electrical signals to sourceregion 104 and drain region 106. In some embodiments, MOSFET 100includes a doped body 120 in substrate 102. Doped body 120 extends fromsource region 102 under spacer 116 and under a portion of variablethickness gate dielectric 112. Doped body also extends below sourceregion 104 in substrate 102. In some embodiments, MOSFET 100 includes alightly doped drain (LDD) region 125 extending from source region 104under spacer 116.

In some embodiments, substrate 102 comprises an elementary semiconductorincluding silicon or germanium in a crystal, a polycrystalline, or anamorphous structure; a compound semiconductor including silicon carbide,gallium arsenic, gallium phosphide, indium phosphide, indium arsenide,and indium antimonide; an alloy semiconductor including SiGe, GaAsP,AlinAs, AlGaAs, GaInAs, GaInP, and GaInAsP; any other suitable material;or combinations thereof. In some embodiments, the alloy semiconductorsubstrate has a gradient SiGe feature in which the Si and Ge compositionchange from one ratio at one location to another ratio at anotherlocation of the gradient SiGe feature. In some embodiments, the alloySiGe is formed over a silicon substrate. In some embodiments, substrate102 is a strained SiGe substrate. In some embodiments, the semiconductorsubstrate includes a doped epi layer or a buried layer. In someembodiments, the compound semiconductor substrate has a multilayerstructure, or the substrate includes a multilayer compound semiconductorstructure.

In some embodiments, substrate 102 is a doped substrate. In someembodiments, substrate 102 is a high resistance substrate.

Source region 104 and drain region 106 are areas of higher chargemobility within substrate 102. In some embodiments, source region 104and drain region 106 have higher hole mobility than substrate 102. Insome embodiments, source region 104 and drain region 106 have higherelectron mobility than substrate 102. In some embodiments, source region104 and drain region 106 include various doping configurations dependingon design requirements. In some embodiments, source region 104 and drainregion 106 are doped with p-type or n-type dopants. For example, sourceregion 104 and drain region 106 are doped with p-type dopants, such asboron or BF₂; n-type dopants, such as phosphorus or arsenic; and/orcombinations thereof. In some embodiments, source region 104 and drainregion 106 are configured for an N-type metal-oxide-semiconductortransistor (referred to as an NMOS) or for a P-typemetal-oxide-semiconductor transistor (referred to as a PMOS).

Variable thickness gate dielectric; layer 112 is positioned between gateelectrode 114 and substrate 102. Variable thickness gate dielectriclayer 112 helps to reduce a capacitance between gate electrode layer 114and drain region 106. Variable thickness gate dielectric layer 112includes a first portion 112 a having a first thickness, a secondportion 112 b having a second thickness, and a third portion 112 chaving a third thickness. The first thickness is less than the secondthickness, and the second thickness is less than the third thickness.Third portion 112 c is closest to drain region 106. In some embodiments,variable thickness gate dielectric layer 112 includes two portionshaving different thicknesses. In some embodiments, variable thicknessgate dielectric; layer 112 has more than three different portions, eachportion having a different thickness. A material of first portion 112 a,second portion 112 b and third portion 112 c is the same. In someembodiments at least one of first portion 112 a second portion 1121 orthird portion 112 c includes a different material from at least oneother of the first portion, the second portion or the third portion. Insome embodiments, a width of each portion of variable thickness gatedielectric layer 112 is equal. In some embodiments, a width of at leastone portion of variable thickness gate dielectric layer 112 is differentfrom at least one other portion of the variable thickness gatedielectric layer. In some embodiments, a ratio between the thickness offirst portion 112 a and the thickness of third portion 112 c ranges fromabout 0.1 to about 0.9.

In some embodiments, variable thickness gate dielectric layer 112 isformed by a thermal oxidation, nitridation, sputter deposition, chemicalvapor deposition, a combination thereof, or another suitable formationprocess. In some embodiments, variable thickness gate dielectric layer112 is formed by a combination of layer formation steps and materialremoval steps. In some embodiments, variable thickness gate dielectriclayer 112 comprises silicon oxide, silicon nitride, nitrified siliconoxide, silicon oxynitride, and high-K (for example, a K>8) dielectrics.The high-K dielectrics include metal oxides, metal silicates, metalnitrides, transition metal-oxides, transition metal silicates, metalaluminates, and transition metal nitrides, or combinations thereof. Forexample, in some embodiments, the high-K dielectrics include, but arenot limited to, one or more of aluminum oxide (Al₂O₃), hafnium ox(HfO₂), hafnium oxynitride hafnium silicate (HfSiO₄) zirconium oxide(ZrO₂), zirconium oxynitride (ZrON), zirconium silicate (ZrSiO₂),yttrium oxide (Y₂O₃), lanthanum oxide (La₂O₃), cerium oxide (CeO₂),titanium oxide (TiO₂), tantalum oxide (Ta₂O₅), or combinations thereof.

Gate electrode layer 114 is configured to receive a control signal toselectively activate and deactivate charge transfer between sourceregion 104 and drain region 106. In some embodiments, gate electrodelayer 114 comprises doped polysilicon and/or metal. In some embodiments,gate electrode layer 114 comprises polysilicon, doped polysilicon,amorphous polysilicon, polysilicon-germanium, combinations thereof, or,in some embodiments, the gate electrode layer 114 includes anothersuitable conductive material.

Spacers 116 are configured to electrically insulate gate electrode layer114. In some embodiments, spacers 116 are formed by a wet etchingprocess, a dry etching process, or combinations thereof. In someembodiments, the dry etching process is an anisotropic dry etchingprocess.

Contacts 130 are configured to provide electrical signals to sourceregion 104 or drain region 106. In some embodiments, contacts 130comprise conductive vias to provide electrical connection between sourceregion 104 and drain region 106 and an interconnect structure. In someembodiments, the conductive vias comprise copper, aluminum, tungsten oranother suitable conductive material. In some embodiments, contacts 130comprise a silicide layer over source region 104 or drain region 106.

Doped body 120 has a higher dopant concentration in comparison withsubstrate 102, but a tower dopant concentration that source region 104.In some embodiments, doped body 120 has a dopant type opposite to adopant type of drain region 106. In some embodiments, doped body 120 isformed during a self-align process. In some embodiments, doped body 120is formed using a single implantation operation. In some embodiments,doped body 120 is formed using multiple implantation operations. In someembodiments, the multiple implantation operations use different dopanttypes and/or different implantation energies. In some embodiments, adopant concentration in a surface portion of doped body 120 adjacent atop surface of substrate 102 ranges from about 1×10¹⁴ ions/cm³ to about1×10¹⁷ ions/cm³. In some embodiments, a depth of the doped body rangesfrom about 0.01 microns to about 0.3 microns.

LDD region 125 has a higher dopant concentration than substrate 102, buta lower dopant concentration than source region 104. LDD region 125 isformed in a portion of substrate 102 under spacer 116 which is adjacentto source region 104. In some embodiments, MOSFET 100 includes anadditional LDD region under spacer 116 adjacent to drain region 106;however, the additional LDD region has a lower dopant concentration thanLDD region 125. In some embodiments, LDD region 125 is formed by an ionimplantation process. A dopant type of LDD region 125 is a same dopanttype as that used in drain region 106. In some embodiments, a dopantconcentration in LDD region 125 ranges from about 1×10¹⁴ ions/cm³ toabout 1×10¹⁷ ions/cm³.

FIG. 1B is a surface dopant concentration profile 150 across MOSFET 100in accordance with one or more embodiments. Dopant concentration profile150 indicates that a dopant concentration under spacer 116 adjacent tosource region 104 is higher than a dopant concentration under spacer 116adjacent to drain region 106. In some embodiments, the dopantconcentration under spacer 116 adjacent to source region 104 is about 10times to about 100 times greater than the dopant concentration underspacer 116 adjacent to drain region 106. In some embodiments, a dopantconcentration under spacer 116 adjacent to drain region 106 ranges fromabout 1×10¹³ ions/cm³ to about 1×10¹⁵ ions/cm³.

A first capacitance Cgs and a second capacitance Cgd impact gate bounceinduced shoot-through in a power management integrated circuit (PMIC).Gate bounce is an increase in a voltage applied to a gate of MOSFET 100during switching on of the MOSFET. If the gate bounce results in thevoltage applied to gate structure 110 of MOSFET 100 exceeding abreak-down voltage of the MOSFET, variable thickness gate dielectriclayer 112 of the MOSFET will be damaged. The first capacitance Cgs andthe second capacitance Cgd also impact a switching loss during operationof MOSFET 100.

The dopant concentration under spacer 116 adjacent to source region 104impacts a first capacitance (Cgs) between gate electrode 114 and thesource region 104. Similarly, the dopant concentration under spacer 116adjacent to drain region 106 impacts a capacitance (Cgd) between gateelectrode 114 and the drain region. In some power management integratedcircuits (PMICs), such as buck converters, a higher Cgs is used toreduce a risk of gate bounce induced shoot-through. Gate bounce inducedshoot-through is an unintentional turning-on of the channel of a MOSFETdue to a voltage swing at a gate of the MOSFET. In contrast a lower Cgdhelps to reduce switching loss. The switching loss impacts the powerdissipated by the MOSFET. If the power dissipation is too great, theMOSFET fails, in some instances, and potentially damages surroundingcircuitry.

The asymmetric character of dopant concentration, such as that ofprofile 150, makes it possible to achieve an advantage in that the riskof gate bounce induced shoot through is reduced by having a higher Cgs,while switching loss is reduced by having a low Cgd. Thus, MOSFET 100having dopant concentration profile 150 exhibits better performance thana MOSFET having a symmetrical dopant concentration profile.

FIG. 2 is a schematic diagram of a converter 200 including MOSFET 100 inaccordance with one or more embodiments. Converter 200 includes a highside transistor 210 configured to receive a supply voltage Vdd.Converter 200 further includes a low side transistor 220 configured toconnect to a ground voltage. An inductor 230 is connected to a nodepositioned between high side transistor 210 and low side transistor 220.High side transistor 210 is a p-type metal oxide semiconductor (PMOS)transistor. In some embodiments, high side transistor 210 is an n-typemetal oxide semiconductor (NMOS) transistor. Low side transistor 220 isan NMOS transistor. In some embodiments, where high side transistor 210is an NMOS transistor, converter 200 has a smaller physical size due toincreased electron mobility in the NMOS transistor in comparison withthe PMOS transistor.

In some embodiments, converter 200 is configured as a buck converter. Insome embodiments where converter 200 is a buck converter, the converteris used as a direct current (DC) to DC step down converter to reduce areceived DC voltage to a lower voltage level. Inductor 230 is configuredto minimize a change in current while changing a voltage which converter200 supplies to a load. In some embodiments, converter 200 is a part ofa PMIC.

FIG. 3A is a cross-section view of an NMOS device 300 in accordance withone or more embodiments. In some embodiments, NMOS device 300 isconfigured to be a low side transistor, e.g., low side transistor 220.NMOS device 300 comprises a substrate 302, a source region 304 in thesubstrate and a drain region 306 in the substrate. Source region 304 isseparated into a p-type doped source region 304 a and an n-type dopedsource region 304 b. A gate structure 310 is positioned on substrate 302between source region 304 and drain region 306. Gate structure 310includes a variable thickness gate dielectric layer 312 over substrate302, a gate electrode layer 314 over the variable thickness gatedielectric layer and spacers 316 on the substrate covering sidewalls ofthe variable thickness gate dielectric layer and the gate electrodelayer. Variable thickness gate dielectric layer 312 is shown as a singlelayer for the sake of simplicity. NMOS device 300 also includes contacts330 configured to provide electrical signals to source region 304 anddrain region 306. NMOS device 300 also includes a p-type doped body 320in substrate 302. P-type doped body 320 extends from source region 304under spacer 316 and under a portion of variable thickness gatedielectric layer 312. P-type doped body 320 extends below source region304 in substrate 302. NMOS device 300 further includes a p-well 340 insubstrate 302. P-well 340 surrounds p-type doped body 320, source region304 and drain region 306. P-well 340 extends between p-type doped body320 and drain region 306. NMOS device 300 further includes isolationregions 350 in substrate 302 configured to electrically separate NMOSdevice 300 from adjacent circuitry.

Substrate 302, source region 304, drain region 306, gate structure 310,and contacts 330 are similar to substrate 102, source region 104, drainregion 106, gate structure 110, and contacts 130, respectively. P-typedoped body 320 comprises p-type dopants. In some embodiments, the p-typedopants include at least one of boron, boron di-fluoride (BF₂), oranother suitable p-type dopant. In some embodiments, p-type doped body320 also includes n-type dopants, such as arsenic, phosphorous, oranother suitable n-type dopant. In some embodiments, p-type doped body320 is formed using a single implantation process. In some embodiments,p-type doped body 320 is formed using multiple implantation processes.

An example of a multiple implantation process for forming p-type dopedbody 320 includes a first implantation using an n-type dopant, such asarsenic. The first implantation includes a dopant concentration rangingfrom about 5×10¹² ions/cm³ to about 1×10¹⁵ ions/cm³. An energy of thefirst implantation process ranges from about 2 kilo electron volts (kEv)to about 60 kEv. A second implantation process uses a p-type dopant,such as boron or BF₂, at a dopant concentration ranging from about1×10¹² ions/cm³ to about 1×10¹⁴ ions/cm³. An implantation energy of thesecond implantation process ranges from about 5 kEv to about 120 kEv.The implantation energy of the second implantation is higher than theimplantation energy of the first implantation process. A thirdimplantation process uses a p-type dopant, such as boron, at a dopantconcentration ranging from about 1×10¹² ions/cm³ to about 1×10¹⁴ions/cm³. An implantation energy of the third implantation processranges from about 10 kEv to about 300 kEv. The implantation energy ofthe third implantation process is higher than the implantation energy ofthe second implantation process. In some embodiments, a dopant used forthe second implantation process is a same dopant as that used in thethird implantation process. In some embodiments, a dopant used for thesecond implantation process is different from that used in the thirdimplantation process.

In some embodiments, p-type doped body 320 is formed by a self-alignedprocess in which variable thickness gate dielectric layer 312 and gateelectrode layer 314 are used as part of a mask during the implantationprocess(es) for forming p-type doped body 320.

P-well 340 is in substrate 102 surrounding p-type doped body 320, sourceregion 304 and drain region 306. P-well 340 includes a p-type dopant. Insome embodiments, the p-type dopant comprises boron, BF₂, aluminum orother suitable p-type dopants. In some embodiments, p-well 340 comprisesan epi-layer grown on substrate 302. In some embodiments, the epi-layeris doped by adding dopants during the epitaxial process. In someembodiments, the epi-layer is doped by ion implantation after theepi-layer is formed. In some embodiments, p-well 340 is formed by dopingsubstrate 302. In some embodiments, the doping is performed by ionimplantation. In some embodiments, p-well 340 has a dopant concentrationranging from 1×10¹² atoms/cm³ to 1×10¹⁴ atoms/cm³.

Isolation regions 350 electrically separate NMOS device 300 fromsurrounding circuitry. In some embodiments, isolation regions 350 areisolation features, such as shallow trench isolation (STI), localoxidation of silicon (LOCOS), or other suitable isolation features. Insome embodiments, isolation regions 350 are undoped portions ofsubstrate 302. In some embodiments, isolation regions 350 are formed byetching substrate 302 to form an opening and filling the opening withnon-conductive material.

NMOS device 300 includes an asymmetric dopant profile due to the lack ofa doped region under spacer 316 adjacent drain region 306. In someembodiments, NMOS device 300 includes a very low dopant concentrationunder spacer 316 adjacent drain region 306. A dopant concentration underspacer 316 adjacent drain region 306 is about 10 to about 100 times lessthan a dopant concentration under spacer 316 adjacent source region 304.

FIG. 3B is a cross-section view of an NMOS device 300′ in accordancewith one or more embodiments. In some embodiments, NMOS device 300′ isconfigured to be a low side transistor, e.g., low side transistor 220.NMOS device 300′ comprises a substrate 302, a source region 304 in thesubstrate and a drain region 306 in the substrate. Source region 304 isseparated into a p-type doped source region 304 a and an n-type dopedsource region 304 b. A gate structure 310 is positioned on substrate 302between source region 304 and drain region 306. Gate structure 310includes a variable thickness gate dielectric layer 312 over substrate302, a gate electrode layer 314 over the gate dielectric layer andspacers 316 on the substrate covering sidewalls of the gate dielectriclayer and the gate electrode layer. NMOS device 300′ also includescontacts 330 configured to provide electrical signals to source region304 and drain region 306. NMOS device 300′ also includes a p-type dopedbody 320 in substrate 302. P-type doped body 320 extends from sourceregion 304 under spacer 316 and under a portion of variable thicknessgate dielectric layer 312. P-type doped body 320 extends below sourceregion 304 in substrate 302. NMOS device 300′ further includes isolationregions 350 in substrate 302 configured to electrically separate NMOSdevice 300 from adjacent circuitry. NMOS device 300′ is different fromNMOS device 300 in that NMOS device 300′ does not include p-well 340.

NMOS device 300′ includes an asymmetric dopant profile due to the lackof a doped region under spacer 316 adjacent drain region 306. In someembodiments, NMOS device 300′ includes a lower dopant concentrationunder spacer 316 adjacent drain region 306. A dopant concentration underspacer 316 adjacent drain region 306 is about 10 to about 100 times lessthan a dopant concentration under spacer 316 adjacent source region 304.

FIG. 3C is a cross-section view of an NMOS device 300″ in accordancewith one or more embodiments. In some embodiments, NMOS device 300″ isconfigured to be a low side transistor, e.g., low side transistor 220.NMOS device 300″ comprises a substrate 302, a source region 304 in thesubstrate and a drain region 306 in the substrate. Source region 304 isseparated into a p-type doped source region 304 a and an n-type dopedsource region 304 b. A gate structure 310 is positioned on substrate 302between source region 304 and drain region 306. Gate structure 310includes a variable thickness gate dielectric layer 312 over substrate302, a gate electrode layer 314 over the gate dielectric layer andspacers 316 on the substrate covering sidewalls of the gate dielectriclayer and the gate electrode layer. NMOS device 300″ includes an LDDregion 325 extending from source region 304 under spacer 316 adjacentthe source region. NMOS device 300″ also includes contacts 330configured to provide electrical signals to source region 304 and drainregion 306. NMOS device 300″ also includes a p-well 340 in substrate302. P-well 340 surrounds source region 304 and drain region 306 andextends beneath variable thickness gate dielectric layer 312. NMOSdevice 300″ further includes isolation regions 350 in substrate 302configured to electrically separate NMOS device 300 from adjacentcircuitry. NMOS device 300″ is different from NMOS device 300 in thatNMOS device 300″ does not include p-type doped body 320, but doesinclude LDD region 325.

In some embodiments, LDD region 325 is formed by an ion implantationprocess. A dopant type of LDD region 325 is an n-type dopant. In someembodiments, a dopant concentration in LDD region 325 ranges from about1×10¹⁴ ions/cm³ to about 1×10¹⁷ ions/cm³. LDD region 325 extends underspacer 316, but does not extend under variable thickness gate dielectriclayer 312.

LDD region 325 is formable by a non-self aligned process. In someembodiments, an anneal process follows the ion implantation process. Insome embodiments, to minimize significant diffusion of dopants, such asboron, arsenic, phosphorus, etc., the peak anneal temperature should beequal to or less than about 1010° C. for rapid thermal anneal (RTA). Insome embodiments, the peak anneal temperature is equal to or less thanabout 900° C. The duration of such RTA, or rapid thermal processing(RTP) anneal, is affected by the anneal temperature. For a higher annealtemperature, the anneal time is kept lower. In some embodiments, the RTAduration is equal to or less than about 60 seconds. For ample, theanneal process is performed at a temperature in a range from about 750°C. to about 850° C. for a duration in a range from about 5 seconds toabout 60 seconds, in accordance with some embodiments. If millisecondanneal (or flash anneal) is used, the peak anneal temperature is higherthan the RTA temperature and the duration is reduced. In someembodiments, the peak anneal temperature is equal to or less than about1250° C. The duration of the millisecond anneal is equal to or less thanabout 40 milliseconds, in accordance with some embodiments.

NMOS device 300″ includes an asymmetric dopant profile due to the lackof a doped region under spacer 316 adjacent drain region 306. In someembodiments, NMOS device 300″ includes a very low dopant concentrationunder spacer 316 adjacent drain region 306. A dopant concentration underspacer 316 adjacent drain region 306 is about 10 to about 100 times lessthan a dopant concentration under spacer 316 adjacent source region 304.

FIG. 3D is a cross-section view of an NMOS device 300* in accordancewith one or more embodiments. In some embodiments, NMOS device 300* isconfigured to be a low side transistor, e.g., low side transistor 220.NMOS device 300* comprises a substrate 302, a source region 304 in thesubstrate and a drain region 306 in the substrate. Source region 304 isseparated into a p-type doped source region 304 a and an n-type dopedsource region 304 b. A gate structure 310 is positioned on substrate 302between source region 304 and drain region 306. Gate structure 310includes a variable thickness gate dielectric layer 312 over substrate302, a gate electrode layer 314 over the gate dielectric layer andspacers 316 on the substrate covering sidewalls of the gate dielectriclayer and the gate electrode layer. NMOS device 300* also includescontacts 330 configured to provide electrical signals to source region304 and drain region 306. NMOS device 300* also includes a p-well 360 insubstrate 302. P-well 340 surrounds source region 304 and extendsbeneath variable thickness gate dielectric layer 312. NMOS device 300*also includes an n-well 370 in substrate 302. N-well 340 surrounds drainregion 306 and extends beneath variable thickness gate dielectric layer312. NMOS device 300* further includes isolation regions 350 insubstrate 302 configured to electrically separate NMOS device 300 fromadjacent circuitry. NMOS device 300* is different from NMOS device 300in that NMOS device 300* does not include p-type doped body 320, butdoes include p-well 360 and n-well 370. In some embodiments, NMOS device300* further includes an LDD region 325 extending from source region 304under spacer 316 adjacent the source region.

In some embodiments, p-well 360 is formed by an ion implantationprocess. In some embodiments, a dopant concentration in p-well 360ranges from about 1×10¹⁵ ions/cm³ to about 1×10¹⁸ ions/cm³. P-well 360extends under spacer 316 and under variable thickness gate dielectriclayer 312.

In some embodiments, n-well 370 is formed by an ion implantationprocess. In some embodiments, a dopant concentration in n-well 370ranges from about 1×10¹⁴ ions/cm³ to about 1×10¹⁷ ions/cm³. N-well 370extends under spacer 316 and under variable thickness gate dielectriclayer 312.

P-well 360 and n-well 370 are formed by a non-self aligned process. Agap between p-well 360 and n-well 370 includes an undoped or lightlydoped portion of substrate 302. The gap is less than a length ofvariable thickness gate dielectric layer 312. In some embodiments,p-well 360 contacts n-well 370 and the gap is omitted. In someembodiments, an anneal process follows the ion implantation process. Tominimize significant diffusion of dopants, such as boron, BF₂, etc., thepeak anneal temperature should be equal to or less than about 1010″ C.for rapid thermal anneal (RTA). The duration of such RTA, or rapidthermal processing (RTP) anneal, is affected by the anneal temperature.For a higher anneal temperature, the anneal time is kept lower. In someembodiments, the RTA duration is equal to or less than about 60 seconds.For example, the anneal process is performed at a temperature in a rangefrom about 750° C. to about 850° C. for a duration in a range from about5 seconds to about 60 seconds, in accordance with some embodiments. Ifmillisecond anneal (or flash anneal) is used, the peak annealtemperature is higher than the RTA temperature and the duration isreduced. In some embodiments, the peak anneal temperature is equal to orless than about 1250° C. The duration of the millisecond anneal is equalto or less than about 40 milliseconds, in accordance with someembodiments.

NMOS device 300* includes an asymmetric dopant profile due to the lackof a doped region under spacer 316 adjacent drain region 306. In someembodiments, NMOS device 300* includes a very low dopant concentrationunder spacer 316 adjacent drain region 306. A dopant concentration underspacer 316 adjacent drain region 306 is about 10 to about 100 times lessthan a dopant concentration under spacer 316 adjacent source region 304.

FIG. 3E is a cross-section view of an NMOS device 300″ in accordancewith one or more embodiments. In some embodiments, NMOS device 300″ isconfigured to be a low side transistor, e.g., low side transistor 220.NMOS device 300″ comprises a substrate 302, a source region 304 in thesubstrate and a drain region 306 in the substrate. Source region 304 isseparated into a p-type doped source region 304 a and an n-type dopedsource region 304 b. A gate structure 310 is positioned on substrate 302between source region 304 and drain region 306. Gate structure 310includes a variable thickness gate dielectric layer 312 over substrate302, a gate electrode layer 314 over the gate dielectric layer andspacers 316 on the substrate covering sidewalls of the gate dielectriclayer and the gate electrode layer. NMOS device 300″ also includescontacts 330 configured to provide electrical signals to source region304 and drain region 306. NMOS device 300″ also includes a p-well 360 insubstrate 302. P-well 340 surrounds source region 304 and extendsbeneath variable thickness gate dielectric layer 312. NMOS device 300″further includes isolation regions 350 in substrate 302 configured toelectrically separate NMOS device 300 from adjacent circuitry. NMOSdevice 300″ differs from NMOS device 300 in that NMOS device 300″ doesnot include p-type doped body 320, but does include p-well 360. In someembodiments, NMOS device 300″ further includes an LDD region 325extending from source region 304 under spacer 316 adjacent the sourceregion.

NMOS device 300″ includes an asymmetric dopant profile due to the lackof a doped region under spacer 316 adjacent drain region 306. In someembodiments, NMOS device 300″ includes a very low dopant concentrationunder spacer 316 adjacent drain region 306. A dopant concentration underspacer 316 adjacent drain region 306 is about 10 to about 100 times lessthan a dopant concentration under spacer 316 adjacent source region 304.

FIG. 4A is a cross-section view of an NMOS device 400 in accordance withone or more embodiments. In some embodiments, NMOS device 400 isconfigured to be a high side transistor, e.g., high side transistor 210.NMOS device 400 is similar to NMOS device 300, like elements have a samereference number increased by 100. NMOS 400 also includes deep n-well480 in substrate 402 beneath p-well 440.

Deep n-well 480 includes n-type dopants such as phosphorous, arsenic, orother suitable n-type dopants. In some embodiments, n-well 480 is formedby ion implantation. An implantation energy of deep n-well 480 is higherthan an implantation energy for p-well 440. In some embodiments, n-well480 is formed by epitaxially growing a layer over substrate 402. In someembodiments, the epitaxially layer is doped following growth of theepitaxial layer. In some embodiments, dopants are mixed with depositiongases during growing of the epitaxial layer. In some embodiments, adopant concentration in deep n-well 480 ranges from about 1×10¹²ions/cm³ to about 3×10¹³ ions/cm³.

FIG. 4B is a cross-section view of an NMOS device 400′ in accordancewith one or more embodiments. In some embodiments, NMOS device 400′ isconfigured to be a high side transistor, e.g., high side transistor 210.NMOS device 400′ is similar to NMOS device 300′, like elements have asame reference number increased by 100. NMOS 400′ also includes deepn-well 480 in substrate 402 beneath p-well 440.

FIG. 4C is a cross-section view of an NMOS device 400{circumflex over( )} in accordance with one or more embodiments. In some embodiments,NMOS device 400{circumflex over ( )} is configured to be a high sidetransistor, e.g., high side transistor 210. NMOS device 400{circumflexover ( )} is similar to NMOS device 300, like elements have a samereference number increased by 100. NMOS 400{circumflex over ( )}includes n-well 490 in substrate 402 in place of p-well 340. N-well 490surrounds p-body 420 and drain region 406 and extends under gatestructure 410. In some embodiments, n-well 490 is formed by a similarprocess as n-well 370.

FIG. 4D is a cross-section view of an NMOS device 400* in accordancewith one or more embodiments. In some embodiments, NMOS device 400* isconfigured to be a high side transistor, e.g., high side transistor 210.NMOS device 400* is similar to NMOS device 300″, like elements have asame reference number increased by 100. NMOS 400* also includes deepn-well 480 in substrate 402 beneath p-well 440.

FIG. 4E is a cross-section view of an NMOS device 400{circumflex over( )} in accordance with one or more embodiments. In some embodiments,NMOS device 400{circumflex over ( )} is configured to be a high sidetransistor, e.g., high side transistor 210. NMOS device 400{circumflexover ( )} is similar to NMOS device 300*, like elements have a samereference number increased by 100. NMOS 400{circumflex over ( )} alsoincludes deep n-well 480 in substrate 402 beneath p-well 440.

FIG. 4F is a cross-section view of an NMOS device 400 ^(#) in accordancewith one or more embodiments. In some embodiments, NMOS device 400 ^(#)is configured to be a high side transistor, e.g., high side transistor210. NMOS device 400 ^(#) is similar to NMOS device 300″, like elementshave a same reference number increased by 100. NMOS 400 ^(#) alsoincludes deep n-well 480 in substrate 402 beneath p-well 440.

FIG. 5A is a cross-section view of a PMOS device 500 in accordance withone or more embodiments. In some embodiments, PMOS device 500 isconfigured to be a high side transistor, e.g., high side transistor 210.PMOS device 500 is similar to NMOS device 400, except dopant types forthe doped body and the well are changed from p-type dopants to n-typedopants. Like elements have a same reference number increased by 100.

In place of p-type doped body 420, PMOS device 500 includes n-type dopedbody 595. N-type doped body 595 is formable using a self-alignedapproach. In some embodiments, n-type doped body 595 is formed by ionimplantation. In some embodiments, n-type doped body 595 is formed by asingle ion implantation. In some embodiments, n-type doped body 595 isformed by a plurality of ion implantation process. For example, a firstimplant process includes doping a p-type dopant, such as boron or BF₂,at a power of 2 keV to 60 keV and a dopant concentration of about 5×10¹²ions/cm³ to about 1×10¹⁵ ions/cm³. A second implant process includesdoping a n-type dopant, such as phosphorous or arsenic, at a power ofabout 5 keV to about 120 keV and a dopant concentration of about 1×10¹²ions/cm³ to about 1×10¹⁴ ions/cm³. A third implant process includesdoping an n-type dopant, such as phosphorous or arsenic, at a power ofabout 10 keV to about 300 keV and a dopant concentration of about 1×10¹²ions/cm³ to about 1×10¹⁴ ions/cm³. In some embodiments, n-type dopedbody 595 is formed using more or less than three ion implantationprocesses.

FIG. 5B is a cross-section view of a PMOS device 500′ in accordance withone or more embodiments. In some embodiments, PMOS device 500′ isconfigured to be a high side transistor, e.g., high side transistor 210.PMOS device 500′ is similar to NMOS device 400′, except dopant type forthe doped body is changed from p-type dopants to n-type dopants. Likeelements have a same reference number increased by 100.

FIG. 5C is a cross-section view of a PMOS device 500″ in accordance withone or more embodiments. In some embodiments, PMOS device 500″ isconfigured to be a high side transistor, e.g., high side transistor 210.PMOS device 500″ is similar to NMOS device 400{circumflex over ( )},except dopant type for the well is changed from p-type dopants to n-typedopants. Like elements have a same reference number increased by 100.

FIG. 5D is a cross-section view of a PMOS device 500* in accordance withone or more embodiments. In some embodiments, PMOS device 500* isconfigured to be a high side transistor, e.g., high side transistor 210.PMOS device 500* is similar to NMOS device 400*, except a location ofthe n-well and p-well are reversed. Like elements have a same referencenumber increased by 100.

FIG. 6 is a flow chart of a method 600 of making a MOSFET in accordancewith one or more embodiments. Method 600 begins with operation 602 inwhich a deep well, e.g. deep n-well 480, is formed. The deep wellcontains n-type dopants, such as arsenic, phosphorous or other suitablen-type dopants. In some embodiments, the deep well is formed byepitaxially growing a layer on a substrate. In some embodiments, dopantsare included in the epitaxial deep well during the growing process. Insome embodiments, the deep well is formed by performing an ionimplantation process. In some embodiments, the ions are implanted intothe substrate to form the deep well. In some embodiments, the ions areimplanted into the epitaxial layer to form the deep well. In someembodiments, operation 602 is omitted. In some embodiments, operation602 is omitted if the MOSFET is a low side transistor in a converter,such as low side NMOS 220.

FIG. 7A is a cross-sectional view of a MOSFET following operation 602 inaccordance with one or more embodiments. An ion implantation process 710is used to form deep well 480 in substrate 402. In some embodiments,isolation structures 450 are already part of the MOSFET during ionimplantation process 710.

Returning to FIG. 6 , method 600 continues with operation 604 in whichan n-well or p-well, e.g., p-well 440, is formed. The p-well containsp-type dopants, such as boron, BF₂ or other suitable p-type dopants. Then-well contains n-type dopants, such as arsenic, phosphorous or othersuitable n-type dopants. In some embodiments, a single p-well or n-wellis formed. In some embodiments, the single p-well or n-well extends overan entire portion of the substrate, as in FIG. 4A. In some embodiments,the single p-well or n-well extends over less than an entire portion ofthe substrate, as in FIG. 4F. In some embodiments, both a p-well and ann-well are formed, as in FIG. 4E. In some embodiments, the p-well orn-well is formed by epitaxially growing a layer on a substrate. In someembodiments, dopants are included in the epitaxial p-well or n-wellduring the growing process. In some embodiments, the p-well or n-well isformed by performing an ion implantation process. In some embodiments,the ions are implanted into the substrate to form the p-well or n-well.In some embodiments, the ions are implanted into the epitaxial layer toform the p-well or n-well. In some embodiments, operation 604 isomitted. In some embodiments, operation 604 is omitted if the MOSFETonly includes a doped body, such a FIG. 4B.

FIG. 7B is a cross-sectional view of the MOSFET following operation 604in accordance with one or more embodiments. An ion implantation process720 is used to form p-well 440 in substrate 402.

Returning to FIG. 6 , a variable thickness gate dielectric layer isformed in operation 606. The variable thickness gate dielectric layer isformed over a top surface of the substrate. FIG. 8 is a flow chart of amethod 800 of forming a variable thickness gate dielectric layer of aMOSFET in accordance with one or more embodiments. In operation 802 adielectric layer is deposited to a first thickness. In some embodiments,the dielectric layer is formed to a maximum thickness of the variablethickness gate dielectric layer, e.g., the thickness of third portion112 c (FIG. 1A). In some embodiments, the dielectric layer is formed toan intermediate thickness of the variable thickness gate dielectriclayer, e.g., the thickness of second portion 112 b. In some embodiments,the dielectric layer is formed by physical vapor deposition (PVD),chemical vapor deposition (CVD); atomic layer deposition (ALD), anepitaxial process or another suitable formation process.

In operation 804, a thickness of a first region of the dielectric layeris reduced to a second thickness. In some embodiments, the first regionincludes all portions of the dielectric layer other than the maximumthickness portion, e.g., first portion 112 a and second portion 112 b(FIG. 1A). In some embodiments, the first region includes less than allportion of the dielectric layer other than the maximum thicknessportion, e.g., only first portion 112 a. In some embodiments, thethickness of the first region of the dielectric layer is reduced usingan etching process, such as a wet etching process or a dry etchingprocess.

In optional operation 806, additional dielectric material is formed overa second region of the dielectric layer up to a third thickness.Operation 806 is omitted in some embodiments which include only tworegions within the variable thickness dielectric layer, e.g., only firstportion 112 and third portion 112 c (FIG. 1A). In some embodiments, thesecond region at least partially overlaps with the first region. In someembodiments, the second region is completely outside the first region.In some embodiments, the third thickness is greater than the firstthickness. In some embodiments, the third thickness is less than thesecond thickness. In some embodiments, the additional dielectricmaterial is formed using a PVD process, a CVD process, an ALD process,an epitaxial process or another suitable formation process. In someembodiments, an additional etching operation is performed followingoptional operation 806 to define another portion of the variablethickness gate dielectric layer. In some embodiments, optional operation806 is repeated more than once to define another portion of the variablethickness gate dielectric layer.

FIG. 7C is a cross-sectional view of the MOSFET following operation 606in accordance with one or more embodiments. Variable thickness gatedielectric layer 412 is formed over a top surface of substrate 402.

Returning to FIG. 6 , a gate electrode layer is formed over the gatedielectric in operation 608. In some embodiments, the gate electrodelayer is formed using a chemical vapor deposition, a physical vapordeposition, electroplating, or another suitable formation process. Insome embodiments, an electrode material is blanket deposited and etchedto form the gate electrode layer. In some embodiments, the gateelectrode layer comprises doped poly silicon and/or metal. In someembodiments, gate electrode layer comprises polysilicon, dopedpolysilicon, amorphous polysilicon, polysilicon-germanium, combinationsthereof, or another suitable conductive material.

FIG. 7D is a cross-sectional view of the MOSFET following operation 608in accordance with one or more embodiments. Gate electrode layer 414 isformed over variable thickness gate dielectric layer 412.

Returning to FIG. 6 , a doped body is implanted into the substrate inoperation 610. The doped body is implantable using a self-alignedprocess. A mask layer is deposited over the substrate and the gateelectrode layer. The mask is patterned so that an edge of the mask isaligned with an edge of the gate electrode layer and the gate dielectriclayer. In some embodiments, the doped body is formed using a singleimplant process. The mask is removed following formation of the dopedbody. In some embodiments, the mask is removed using an ashing processor another suitable mask removal process. In some embodiments, the dopedbody is formed using a plurality of implant processes. In someembodiments, operation 610 is omitted. Operation 610 is omitted if theMOSFET is formed using a non-self aligned approach.

In some embodiments, the doped body is a p-type doped body. In someembodiments, the p-type doped body is formed using three p-body implantprocesses. The first p-body implant process includes doping an n-typedopant, such as arsenic at a power of 2 keV to 60 keV and a dopantconcentration of about 5×10¹² ions/cm³ to about 1×10¹⁵ ions/cm³. Thesecond p-body implant process includes doping a p-type dopant, such asboron or BF₂, at a power of about 5 keV to about 120 keV and a dopantconcentration of about 1×10¹² ions/cm³ to about 1×10¹⁴ ions/cm³. Thethird p-body implant process includes doping a p-type dopant, such asboron or BF₂, at a power of about 10 keV to about 300 keV and a dopantconcentration of about 1×10¹² ions/cm³ to about 1×10¹⁴ ions/cm³. Thefirst p-body implant is the shallowest of the implant processes whilethe third p-body implant is the deepest of the implant processes. Insome embodiments, forming a p-type doped body implant includes more orless than three implant processes.

In some embodiments, the doped body is an n-type doped body. In someembodiments, the n-type doped body is formed using three n-body implantprocesses. The first n-body implant process includes doping a p-typedopant, such as boron or BF₂, at a power of 2 keV to 60 keV and a dopantconcentration of about 5×10¹² ions/cm³ to about 1×10¹⁵ ions/cm³. Thesecond n-body implant process includes doping a n-type dopant, such asphosphorous or arsenic, at a power of about 5 keV to about 120 keV and adopant concentration of about 1×10¹² ions/cm³ to about 1×10¹⁴ ions/cm³.The third n-body implant process includes doping an n-type dopant, suchas phosphorous or arsenic, at a power of about 10 keV to about 300 keVand a dopant concentration of about 1×10¹² ions/cm³ to about 1×10¹⁴ions/cm³. The first n-body implant is the shallowest of the implantprocesses while the third n-body implant is the deepest of the implantprocesses. In some embodiments, forming an n-type doped body implantincludes more or less than three implant processes.

FIG. 7E is a cross-sectional view of the MOSFET following operation 610in accordance with one or more embodiments. P-type doped body 420 isformed in substrate 402 and extends under variable thickness gatedielectric layer 412.

Returning to FIG. 6 , method 600 continues with operation 612 in whichan LDD is implanted in the substrate. In some embodiments, the LDD isformed by an ion implantation process. A dopant type of the LDD is asame dopant type as that used in a drain region. In some embodiments, adopant concentration in the LDD ranges from about 1×10¹⁴ ions/cm³ toabout 1×10¹⁷ ions/cm³. In some embodiments, operation 612 is omitted.Operation 612 is omitted if the MOSFET is formed using a self-alignedapproach. In some embodiments, operation 612 is omitted in non-selfaligned approaches which have an n-well or p-well which extends overless than an entire portion of the substrate.

FIG. 7F is a cross-sectional view of the MOSFET following operation 612in accordance with one or more embodiments. FIG. 7F does not include thep-type doped by of FIG. 7E because the doped body is not included inMOSFETs formed by non-self aligned approaches and LDDs are not formed inMOSFETs formed by self aligned approaches. LDD 425 is formed insubstrate 402 and does not extend under variable thickness gatedielectric layer 412.

Returning to FIG. 6 , spacers are formed in operation 614. The spacersare formed over sidewalls of the gate dielectric layer and the gateelectrode layer. In some embodiments, the spacers are formed by a wetetching process, a dry etching process, or combinations thereof. In someembodiments, the dry etching process is an anisotropic dry etchingprocess.

FIG. 7G is a cross-sectional view of the MOSFET following operation 614in accordance with one or more embodiments. Spacers 416 are formed oversidewalls of variable thickness gate dielectric layer 412 and gateelectrode layer 414. FIG. 7G is directed to an embodiment which includesoperation 610, but does not include operation 612. P-type doped body 420extends under spacer 416 and under a portion of variable thickness gatedielectric layer 412. In embodiments which include operation 612, theLDD extends under spacer 416 but not under variable thickness gatedielectric layer 412.

Returning to FIG. 6 , method 600 continues with operation 616 in whichsource and drains are implanted in the substrate. In some embodiments,the source and drain are doped with p-type or n-type dopants. Forexample, the source and drain are doped with p-type dopants, such asboron or BF₂; n-type dopants, such as phosphorus or arsenic; and/orcombinations thereof. In some embodiments the source and drain areconfigured for an NMOS or for a PMOS. In some embodiments, a dopantconcentration of the source and drain ranges from about 1×10¹⁷ ions/cm³to about 1×10¹⁹ ions/cm³.

FIG. 7H is a cross-sectional view of the MOSFET following operation 616in accordance with one or more embodiments. Source region 404 is formedin substrate 402 in p-type doped body 420. Drain region 406 is formed insubstrate 402 on an opposite side of gate structure 410 from sourceregion 404.

Returning to FIG. 6 , the MOSFET is annealed in operation 618. In someembodiments, the annealing process is used to activate the dopantsimplanted in the previous operations. In some embodiments, the annealingprocess is used to facilitate movement of dopants through the substrate.In some embodiments, the anneal is a rapid thermal anneal, a microsecondanneal or another suitable annealing process. In some embodiments,operation 618 is separated into several operations which are performedafter each implantation process. In some embodiments, operation 618 isseparated into several operations which are performed after selectedimplant processes.

Method 600 continues with operation 620 in which back end processing isperformed. In some embodiments, back end processing includes formationof an inter-layer dielectric (ILD) layer on the substrate. Contact holesare formed in the ILD layer. In some embodiments, the contact holes areformed by etching process, such as dry etching or wet etching, or othersuitable material removal processes. Conductive contacts are formed inthe contact holes to provide electrical connection to the heavily dopedregions in the device. In some embodiments, the conductive contactscomprise copper, aluminum, tungsten, a conductive polymer or anothersuitable conductive material. In some embodiments, a conductive contactis formed in electrical connection with the gate structure. In someembodiments, additional interconnect structures are formed over ILDlayer to provide electrical connections between the source and the drainand other circuitry. In some embodiments, the interconnect structuresprovide electrical connections between the gate structure and othercircuitry.

An aspect of this description relates to a semiconductor device. Thesemiconductor device includes a substrate. The semiconductor devicefurther includes a gate structure over the substrate. The semiconductordevice further includes a source in the substrate on a first side of thegate structure. The semiconductor device further includes a drain in thesubstrate on a second side of the gate structure. The semiconductordevice further includes a first well having a first dopant type, whereinthe first well contacts at least two surfaces of the source. Thesemiconductor device further includes a second well having the firstdopant type, wherein the second well contacts at least two surfaces ofthe drain. The semiconductor device further includes a deep well belowthe first well and below the second well, wherein the second wellextends between the first well and the deep well. In some embodiments,the deep well has a second dopant type, and the second dopant type isopposite the first dopant type. In some embodiments, the second wellcontacts at least two surfaces of the first well. In some embodiments,the gate structure includes a variable thickness gate dielectric layer.In some embodiments, an uppermost surface of the second well between thedrain and the first well is coplanar with a top surface of thesubstrate. In some embodiments, an uppermost surface of a portion of thefirst well under the gate structure is coplanar with a top surface ofthe substrate. In some embodiments, the first well includes a differentdopant species from a dopant species of the second well.

An aspect of this description relates to a semiconductor device. Thesemiconductor device includes a source in a substrate. The semiconductordevice further includes a drain in the substrate, wherein the drain isseparated from the source. The semiconductor device further includes afirst well in the substrate, wherein the first well comprises a firstdopant having a first dopant type, and the first well contacts at leasttwo surfaces of the source. The semiconductor device further includes asecond well in the substrate, wherein the second well comprises a seconddopant having the first dopant type, and the second well physicallycontacts at least two surfaces of the drain. The semiconductor devicefurther includes a deep well having a depth greater than the first welland the second well, wherein the second well is between the drain andthe deep well. In some embodiments, the source includes a split source.In some embodiments, the semiconductor device further includes a lightlydoped drain (LDD) region between the source and the drain. In someembodiments, the first well is separated from the second well. In someembodiments, the deep well directly contacts both the first well and thesecond well. In some embodiments, the semiconductor device furtherincludes a gate structure over the substrate. In some embodiments, thegate structure overlaps a first portion of the first well and a secondportion of the second well. In some embodiments, a portion of thesubstrate under the gate structure is free of the first well and thesecond well.

An aspect of this description relates to a semiconductor device. Thesemiconductor device includes a source in a substrate. The semiconductordevice further includes a drain in the substrate, wherein the drain isseparated from the source. The semiconductor device further includes afirst well in the substrate, wherein the first well includes a firstdopant having a first dopant type, and the first well contacts at leasttwo surfaces of the source. The semiconductor device further includes asecond well in the substrate, wherein the second well includes a seconddopant having the first dopant type, the second well physically contactsat least two surfaces of the drain, and the second well physicallycontacts at least two surfaces of the first well. In some embodiments, adepth of the second well is greater than a depth of the first well. Insome embodiments, the semiconductor device further includes a gatestructure over the substrate. In some embodiments, the gate structureoverlaps a first portion of the first well and a second portion of thesecond well. In some embodiments, an entirety of the substrate betweenthe drain and the first well is occupied by the second well.

It will be readily seen by one of ordinary skill in the art that thedisclosed embodiments fulfill one or more of the advantages set forthabove. After reading the foregoing specification, one of ordinary skillwill be able to affect various changes, substitutions of equivalents andvarious other embodiments as broadly disclosed herein. It is thereforeintended that the protection granted hereon be limited only by thedefinition contained in the appended claims and equivalents thereof.

What is claimed is:
 1. A semiconductor device comprising: a substrate; agate structure over the substrate; a source in the substrate on a firstside of the gate structure; a drain in the substrate on a second side ofthe gate structure; a first well having a first dopant type, wherein thefirst well contacts at least two surfaces of the source; a second wellhaving the first dopant type, wherein the second well contacts at leasttwo surfaces of the drain; and a deep well below the first well andbelow the second well, wherein the second well extends between the firstwell and the deep well.
 2. The semiconductor device of claim 1, whereinthe deep well has a second dopant type, and the second dopant type isopposite the first dopant type.
 3. The semiconductor device of claim 1,wherein the second well contacts at least two surfaces of the firstwell.
 4. The semiconductor device of claim 1, wherein the gate structurecomprises a variable thickness gate dielectric layer.
 5. Thesemiconductor device of claim 1, wherein an uppermost surface of thesecond well between the drain and the first well is coplanar with a topsurface of the substrate.
 6. The semiconductor device of claim 1,wherein an uppermost surface of a portion of the first well under thegate structure is coplanar with a top surface of the substrate.
 7. Thesemiconductor device of claim 1, wherein the first well comprises adifferent dopant species from a dopant species of the second well.
 8. Asemiconductor device comprising: a source in a substrate; a drain in thesubstrate, wherein the drain is separated from the source; a first wellin the substrate, wherein the first well comprises a first dopant havinga first dopant type, and the first well contacts at least two surfacesof the source; a second well in the substrate, wherein the second wellcomprises a second dopant having the first dopant type, and the secondwell physically contacts at least two surfaces of the drain; a deep wellhaving a depth greater than the first well and the second well, whereinthe second well is between the drain and the deep well.
 9. Thesemiconductor device of claim 8, wherein the source comprises a splitsource.
 10. The semiconductor device of claim 8, further comprising alightly doped drain (LDD) region between the source and the drain. 11.The semiconductor device of claim 8, wherein the first well is separatedfrom the second well.
 12. The semiconductor device of claim 8, whereinthe deep well directly contacts both the first well and the second well.13. The semiconductor device of claim 8, further comprising a gatestructure over the substrate.
 14. The semiconductor device of claim 13,wherein the gate structure overlaps a first portion of the first welland a second portion of the second well.
 15. The semiconductor device ofclaim 13, wherein a portion of the substrate under the gate structure isfree of the first well and the second well.
 16. A semiconductor devicecomprising: a source in a substrate; a drain in the substrate, whereinthe drain is separated from the source; a first well in the substrate,wherein the first well comprises a first dopant having a first dopanttype, and the first well contacts at least two surfaces of the source;and a second well in the substrate, wherein the second well comprises asecond dopant having the first dopant type, the second well physicallycontacts at least two surfaces of the drain, and the second wellphysically contacts at least two surfaces of the first well.
 17. Thesemiconductor device of claim 16, wherein a depth of the second well isgreater than a depth of the first well.
 18. The semiconductor device ofclaim 16, further comprising a gate structure over the substrate. 19.The semiconductor device of claim 18, wherein the gate structureoverlaps a first portion of the first well and a second portion of thesecond well.
 20. The semiconductor device of claim 16, wherein anentirety of the substrate between the drain and the first well isoccupied by the second well.